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Abstract:

A positive electrode active material with least part of a surface coated
with a surface treatment layer composed of a phosphate compound. The
phosphate compound contains at least one element selected from the group
consisting of neodymium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

Claims:

1. A non-aqueous electrolyte secondary battery comprising: an electrode
body including a positive electrode in which a positive electrode active
material layer containing a positive electrode active material is formed
on a surface of a positive electrode current collector, a negative
electrode, and a separator placed between the positive electrode and the
negative electrode; a non-aqueous electrolyte; and a casing that
accommodates the electrode body and the non-aqueous electrolyte, wherein
at least part of a surface of the positive electrode active material is
coated with a surface treatment layer composed of a phosphate compound,
and the phosphate compound contains at least one element selected from
the group consisting of neodymium, samarium, europium, gadolinium,
terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium.

2. The non-aqueous electrolyte secondary battery according to claim 1,
wherein the at least one element contained in the phosphate compound is
at least one element selected from the group consisting of neodymium,
samarium, europium, erbium, ytterbium, and lutetium.

3. The non-aqueous electrolyte secondary battery according to claim 2,
wherein the ratio of the phosphate compound on an elemental neodymium,
samarium, europium, erbium, ytterbium, or lutetium basis relative to the
positive electrode active material is 0.010 mass % or more and 0.25 mass
% or less.

4. The non-aqueous electrolyte secondary battery according to claim 1,
wherein the positive electrode active material is a lithium transition
metal oxide having a layered structure.

5. The non-aqueous electrolyte secondary battery according to claim 2,
wherein the positive electrode active material is a lithium transition
metal oxide having a layered structure.

6. The non-aqueous electrolyte secondary battery according to claim 3,
wherein the positive electrode active material is a lithium transition
metal oxide having a layered structure.

7. A method for producing a non-aqueous electrolyte secondary battery,
comprising: coating at least part of a surface of a positive electrode
active material with a surface treatment layer composed of a phosphate
compound by adding a phosphate and a salt containing at least one element
selected from the group consisting of neodymium, samarium, europium,
gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and
lutetium to a solution containing the positive electrode active material;
preparing a positive electrode by forming a positive electrode active
material layer on a surface of a positive electrode current collector,
the positive electrode active material layer containing a positive
electrode active material that is coated with the surface treatment layer
composed of the phosphate compound; preparing an electrode body by
placing a separator between the positive electrode and a negative
electrode; and placing the electrode body and a non-aqueous electrolyte
in a casing.

8. The method according to claim 7, wherein, in the coating the surface
of the positive electrode active material with the surface treatment
layer, an acid or a base is added to the solution containing the positive
electrode active material to control pH of the solution to be 2 to 7.

11. The method according to claim 7, wherein the at least one element
contained in the phosphate compound is at least one element selected from
the group consisting of neodymium, samarium, europium, erbium, ytterbium,
and lutetium.

12. The method according to claim 11, wherein the ratio of the phosphate
compound on the coated positive electrode active material on an elemental
neodymium, samarium, europium, erbium, ytterbium, or lutetium basis
relative to the positive electrode active material is 0.010 mass % or
more and 0.25 mass % or less.

13. The method according to claim 7, wherein the positive electrode
active material is a lithium transition metal oxide having a layered
structure.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present invention contains subject matter related to Japanese
Patent Application No. 2010-55874 filed in the Japan Patent Office on
Mar. 12, 2010, the entire contents of which are incorporated herein by
reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an improvement in a non-aqueous
electrolyte secondary battery and a method for producing the secondary
battery. In particular, the present invention relates to a non-aqueous
electrolyte secondary battery that can achieve high reliability in a
battery having high capacity and a method for producing the secondary
battery.

[0004] 2. Description of Related Art

[0005] In recent years, mobile information terminals such as cellular
phones, notebook computers, and personal digital assistants (PDAs) have
been rapidly decreasing in size and weight. With such a trend, a further
increase in the capacity of batteries serving as driving power sources of
such mobile information terminals has been demanded. To meet such a
demand, a non-aqueous electrolyte secondary battery that uses an alloy
which can occlude and release lithium ions, a carbon material, or the
like as a negative electrode active material and uses lithium transition
metal complex oxide as a positive electrode active material has been
receiving attention as a battery having high energy density.

[0006] An increase in the capacity of existing non-aqueous electrolyte
secondary batteries has been achieved by decreasing the thickness of a
component such as a battery can, a separator, or a current collector
(aluminum foil or copper foil) that is unrelated to capacity or by
achieving a high packing density of an active material (by improving the
packing density of an electrode). However, even if these means for
increasing the capacity are employed, the capacity of non-aqueous
electrolyte secondary batteries cannot be increased markedly. It is
considered that by increasing the charge cut-off voltage, the capacity
and energy density are increased. However, in the case where the charge
cut-off voltage is increased, a positive electrode active material is
degraded and an electrolyte is decomposed by oxidation, which poses a
problem in that the battery characteristics are degraded.

[0007] In view of the foregoing, the surface treatment and the
stabilization of a structure of a positive electrode active material have
been actively researched. For example, there have been the following
proposals.

[0008] (1) In the case where the charge cut-off voltage is increased, the
stabilization of a structure of a positive electrode active material is
achieved by incorporating a Zr element into a lithium transition metal
oxide that has a layered structure and contains lithium and cobalt (see
Japanese Patent No. 4307962 (Patent Document 1)).

[0009] (2) A technique in which at least part of a surface of a positive
electrode active material is coated with a surface treatment layer
composed of a phosphate compound represented by MPOk (M is at least
one trivalent element and k is an integer of 2 to 4). According to this
technique, by suppressing the reaction between an electrolyte and a
positive electrode, the cycle characteristics are improved without
decreasing the initial efficiency (see Japanese Published Unexamined
Patent Application No. 2005-243301 (Patent Document 2)).

[0010] (3) A battery having good high-temperature swelling characteristics
(that is, not swell even at high temperature) by using a positive
electrode active material obtained as follows. An active material
precursor is added dropwise to a coating solution obtained by mixing a
phosphorus compound having a double bond such as
(NH4)2HPO4, a compound containing Al such as
Al(NO3)3.9H2O, and water. A lithium source is added
thereto and heat treatment is performed to obtain the positive electrode
active material (see Japanese Published Unexamined Patent Application No.
2005-166656 (Patent Document 3)).

[0011] However, in the technique disclosed in Patent Document 1, when the
charge cut-off voltage is increased, the structure of a positive
electrode active material can be stabilized to some degree, but the
degree of the stabilization is insufficient. Thus, the battery capacity
is significantly reduced when a battery is stored at high temperature.
The techniques disclosed in Patent Documents 2 and 3 provide a structure
in which a positive electrode active material is coated with a compound
of aluminum or lanthanum. In the case where the charge cut-off voltage is
4.2 V, the effects achieved by such a structure are produced to some
extent. However, in the case where the charge cut-off voltage is further
increased (e.g., the charge cut-off voltage is increased to 4.4 V), the
above-described effects are not sufficiently produced.

BRIEF SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a non-aqueous
electrolyte secondary battery whose storage characteristics at high
temperature can be increased in the case where the charge cut-off voltage
is increased to achieve high battery capacity.

[0013] To achieve the object, an aspect of the present invention provides
a non-aqueous electrolyte secondary battery including an electrode body
including a positive electrode in which a positive electrode active
material layer containing a positive electrode active material is formed
on a surface of a positive electrode current collector, a negative
electrode, and a separator placed between the positive electrode and the
negative electrode; a non-aqueous electrolyte; and a casing that
accommodates the electrode body and the non-aqueous electrolyte, wherein
at least part of a surface of the positive electrode active material is
coated with a surface treatment layer composed of a phosphate compound,
and the phosphate compound contains at least one element selected from
the group consisting of neodymium (Nd), samarium (Sm), europium (Eu),
gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium
(Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0014] In the above-described structure, since at least part of the
surface of the positive electrode active material is coated with the
surface treatment layer, the reaction area between the positive electrode
active material and an electrolyte is decreased. In addition, it is
believed that, since the surface treatment layer is composed of a
phosphate compound containing a limited element such as neodymium and the
phosphate compound is different from a phosphate compound containing
aluminum or lanthanum, the surface treatment layer specifically exhibits
an anticatalyst effect. Thus, the reaction between the positive electrode
active material and an electrolyte can be suppressed. Accordingly, since
the reaction area between the positive electrode active material and the
electrolyte can be decreased while at the same time the reaction
therebetween can be suppressed, the storage characteristics at high
temperature can be improved markedly in the case where the charge cut-off
voltage is increased.

[0015] Furthermore, since the surface treatment layer specifically
exhibits an anticatalyst effect and thus the reaction between the
positive electrode active material and the electrolyte is suppressed, the
above-described effect is produced without impairing the load
characteristics. Normally, when the area of the surface treatment layer
that coats the surface of the positive electrode active material is
increased, an effect of suppressing the reaction between the positive
electrode active material and the electrolyte is sufficiently produced.
However, the load characteristics are degraded because the reaction area
between the positive electrode active material and the electrolyte is
decreased. On the other hand, when the area of the surface treatment
layer that coats the surface of the positive electrode active material is
decreased, the reaction area between the positive electrode active
material and the electrolyte is not so decreased. Therefore, the
degradation of the load characteristics is suppressed, but an effect of
suppressing the reaction between the positive electrode active material
and the electrolyte is not sufficiently produced. That is, the storage
characteristics at high temperature and the load characteristics are in a
tradeoff relationship. However, in the above-described structure, the
effect of suppressing the reaction between the positive electrode active
material and the electrolyte is produced by coating only a part of the
surface of the positive electrode active material. Therefore, the storage
characteristics at high temperature can be improved without impairing the
load characteristics.

[0016] The element contained in the phosphate compound is preferably at
least one element selected from the group consisting of neodymium,
samarium, europium, erbium, ytterbium, and lutetium.

[0017] The ratio of the phosphate compound on an elemental neodymium,
samarium, europium, erbium, ytterbium, or lutetium basis relative to the
positive electrode active material is preferably 0.010 mass % or more and
0.25 mass % or less.

[0018] If the ratio is less than 0.010 mass %, the effect is not
sufficiently produced because the amount of phosphate compound is
excessively small. On the other hand, if the ratio is more than 0.25 mass
%, the surface of the positive electrode active material is excessively
coated with the surface treatment layer. As a result, good storage
characteristics at high temperature are achieved, but sufficient initial
capacity is not obtained and load characteristics are degraded.

[0019] Any material can be used as the positive electrode active material
used in the present invention as long as the material can occlude and
release lithium and has a noble potential. For example, a lithium
transition metal complex oxide having a layered structure, a spinel
structure, or an olivine structure can be used. In addition to lithium
cobaltate, specific examples of the lithium transition metal complex
oxide include lithium complex oxides containing nickel such as a lithium
complex oxide of nickel-cobalt-manganese, a lithium complex oxide of
nickel-aluminum-manganese, and a lithium complex oxide of
nickel-cobalt-aluminum.

[0020] Among the positive electrode active materials, a lithium transition
metal oxide having a layered structure is preferably used. A lithium
transition metal oxide having a layered structure provides high discharge
capacity but poor thermal stability. Thus, by coating the positive
electrode active material with the surface treatment layer composed of a
phosphate compound, thermal stability can be improved while at the same
time high discharge capacity can be achieved.

[0021] The positive electrode active material may be used alone or may be
used by being mixed with other positive electrode active materials.

[0022] An aspect of the present invention provides a method for producing
a non-aqueous electrolyte secondary battery, including a step of coating
at least part of a surface of a positive electrode active material with a
surface treatment layer composed of a phosphate compound by adding a
phosphate and a salt containing at least one element selected from the
group consisting of neodymium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, erbium, thulium, ytterbium, and lutetium to a
solution containing the positive electrode active material; a step of
preparing a positive electrode by forming a positive electrode active
material layer on a surface of a positive electrode current collector,
the positive electrode active material layer containing a positive
electrode active material that is coated with the surface treatment layer
composed of the phosphate compound; a step of preparing an electrode body
by placing a separator between the positive electrode and a negative
electrode; and a step of placing the electrode body and a non-aqueous
electrolyte in a casing.

[0023] By this production method, the above-described non-aqueous
electrolyte secondary battery can be produced.

[0024] In the step of coating the surface of the positive electrode active
material with the surface treatment layer, an acid or a base is
preferably added to the solution containing the positive electrode active
material to control pH of the solution to be 2 to 7.

[0025] If the pH is less than 2, the phosphate compound is dissolved and
thus the surface of the positive electrode active material is sometimes
not coated with the surface treatment layer. If the pH is more than 7,
not only the phosphate compound but also a hydroxide may be deposited,
and the effects unique to a phosphate compound are not sufficiently
produced.

[0027] The salts above are listed as the phosphate, but the phosphate is
not limited thereto. Any phosphate can be used as long as the phosphate
is soluble in water.

(Other Points)

[0028] (1) Any material can be used as the negative electrode active
material used in the present invention as long as the material can
occlude and release lithium. Examples of the negative electrode active
material include carbon materials such as graphite and coke, metal oxides
such as tin oxide, metals such as silicon and tin that can occlude
lithium by being alloyed with lithium, and metallic lithium. Among these
materials, a carbon material such as graphite is preferably used because
such a material has little volume change caused when occluding and
releasing lithium and good reversibility.

[0029] (2) A solvent that has been conventionally used for non-aqueous
electrolyte secondary batteries can be employed as a solvent of a
non-aqueous electrolyte used in the present invention. Preferable
examples of the solvent include carbonic acid ester-based solvents such
as ethylene carbonate (EC), propylene carbonate (PC), gamma-butyrolactone
(GBL), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), and
dimethyl carbonate (DMC); and carbonate-based solvents obtained by
replacing part of hydrogen atoms (H) of these carbonates with fluorine
atoms (F). Furthermore, a solvent obtained by combining a cyclic carbonic
acid ester with a chain carbonic acid ester is particularly preferred.

[0030] Examples of a solute of the non-aqueous electrolyte include
LiPF6, LiBF4, LiN(SO2CF3)2,
LiN(SO2C2F5)2, and
LiPF6-x(CnF2n-1)x (where 1<X<6 and n=1 or 2).
These materials can be used alone or in combination. The concentration of
the solute is not particularly limited, and is preferably in the range of
0.8 to 1.5 moles per 1 liter of the electrolyte.

[0031] (3) Examples of the acid used when pH is adjusted include nitric
acid, sulfuric acid, and hydrochloric acid. Examples of the base include
sodium hydroxide and potassium hydroxide.

[0032] According to the present invention, in the case where the charge
cut-off voltage is increased to achieve high battery capacity, the
storage characteristics at high temperature can be markedly increased.

DETAILED DESCRIPTION OF THE INVENTION

[0033] A non-aqueous electrolyte secondary battery according to the
present invention will now be described. The non-aqueous electrolyte
secondary battery of the present invention is not limited to the
embodiments described below, and can be suitably modified within the
scope of the present invention.

(Preparation of Positive Electrode)

[0034] First, 500 g of lithium cobaltate (represented by LiCoO2, and
including 1.5 mol % Al and Mg dissolved therein and 0.05 mol % Zr
attached on the surface thereof), which was a positive electrode active
material, was added to 3 L of pure water and the mixture was stirred. An
aqueous solution obtained by dissolving 0.77 g of disodium hydrogen
phosphate dodecahydrate and 0.90 g of neodymium nitrate hexahydrate was
added to the mixture being stirred to deposit neodymium phosphate on the
surface of lithium cobaltate. Herein, 10 mass % nitric acid and 10 mass %
sodium hydroxide solution were suitably added to the resultant solution
to keep pH of the solution at 6.5. After such a state was maintained,
suction filtration and washing with water were performed. The resultant
powder was dried at 120° C. and then fired in the air at
300° C. for 5 hours to obtain lithium cobaltate having a surface
treatment layer composed of neodymium phosphate formed on the surface
thereof. By observing the positive electrode active material with a
scanning electron microscope (SEM), it was confirmed that neodymium
phosphate was attached to the surface of lithium cobaltate in a dispersed
form. The amount of neodymium phosphate attached to lithium cobaltate was
0.059 mass % on an elemental neodymium basis.

[0035] Subsequently, the positive electrode active material, acetylene
black (AB) serving as a conducting agent, and polyvinylidene fluoride
(PVDF) serving as a binding agent were mixed so as to have a mass ratio
of 95:2.5:2.5. The resultant mixture was kneaded together with
N-methyl-pyrrolidone (NMP) serving as a solvent to prepare positive
electrode active material slurry. The positive electrode active material
slurry was applied to both sides of a positive electrode current
collector composed of aluminum foil, and drying and rolling were
performed to prepare a positive electrode. The packing density of the
positive electrode active material was 3.6 g/cc.

(Preparation of Negative Electrode)

[0036] Graphite serving as a negative electrode active material,
styrene-butadiene rubber (SBR) serving as a binding agent, and
carboxymethyl cellulose (CMC) serving as a thickener were mixed so as to
have a mass ratio of 98:1:1. The resultant mixture was kneaded in an
aqueous solution to prepare a negative electrode active material slurry.
The negative electrode active material slurry was applied to both sides
of a negative electrode current collector composed of copper foil, and
drying and rolling were performed to prepare a negative electrode. The
packing density of the negative electrode active material was 1.7 g/cc.

(Preparation of Non-Aqueous Electrolyte)

[0037] LiPF6 was added to a solvent obtained by mixing ethylene
carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 3:7 so
that the LiPF6 concentration was 1.0 mol/L. Furthermore, vinylene
carbonate (VC) was added to the solvent in a concentration of 1 mass %
relative to the solvent, and thus a non-aqueous electrolyte was prepared.

(Assembling of Battery)

[0038] After lead terminals were attached to the positive electrode and
the negative electrode, the positive electrode and the negative electrode
were wound with a separator therebetween. This wound body was pressed
into a flat shape, whereby an electrode body was prepared. This electrode
body was inserted into an aluminum laminate serving as a battery casing
and then the non-aqueous electrolyte was injected into the aluminum
laminate to produce a test battery. When the battery was charged to 4.4
V, the design capacity thereof was 750 mAh.

EXAMPLES

First Example

Example 1

[0039] In Example 1, a battery was produced by the same method as that in
the detailed description of the invention.

[0040] The thus-produced battery is hereinafter referred to as an
invention battery A1.

Example 2

[0041] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
samarium phosphate was uniformly dispersed and attached to the surface of
lithium cobaltate by using samarium nitrate hexahydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of samarium phosphate on an
elemental samarium basis relative to lithium cobaltate was 0.062 mass %.
On a molar basis, the amount of samarium phosphate was the same as that
of neodymium phosphate in Example 1.

[0042] The thus-produced battery is hereinafter referred to as an
invention battery A2.

Example 3

[0043] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
europium phosphate was uniformly dispersed and attached to the surface of
lithium cobaltate by using europium nitrate hexahydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of europium phosphate on an
elemental europium basis relative to lithium cobaltate was 0.063 mass %.
On a molar basis, the amount of europium phosphate was the same as that
of neodymium phosphate in Example 1.

[0044] The thus-produced battery is hereinafter referred to as an
invention battery A3.

Example 4

[0045] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which erbium
phosphate was uniformly dispersed and attached to the surface of lithium
cobaltate by using erbium nitrate pentahydrate instead of neodymium
nitrate hexahydrate when a phosphate compound was deposited on the
surface of lithium cobaltate. The ratio of erbium phosphate on an
elemental erbium basis relative to lithium cobaltate was 0.070 mass %. On
a molar basis, the amount of erbium phosphate was the same as that of
neodymium phosphate in Example 1.

[0046] The thus-produced battery is hereinafter referred to as an
invention battery A4.

Example 5

[0047] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
ytterbium phosphate was uniformly dispersed and attached to the surface
of lithium cobaltate by using ytterbium nitrate trihydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of ytterbium phosphate on an
elemental ytterbium basis relative to lithium cobaltate was 0.071 mass %.
On a molar basis, the amount of ytterbium phosphate was the same as that
of neodymium phosphate in Example 1.

[0048] The thus-produced battery is hereinafter referred to as an
invention battery A5.

Example 6

[0049] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
lutetium phosphate was uniformly dispersed and attached to the surface of
lithium cobaltate by using lutetium nitrate trihydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of lutetium phosphate on an
elemental lutetium basis relative to lithium cobaltate was 0.071 mass %.
On a molar basis, the amount of lutetium phosphate was the same as that
of neodymium phosphate in Example 1.

[0050] The thus-produced battery is hereinafter referred to as an
invention battery A6.

Comparative Example 1

[0051] A battery was produced by the same method as that in Example 1,
except that neodymium phosphate was not attached to the surface of
lithium cobaltate.

[0052] The thus-produced battery is hereinafter referred to as a
comparative battery Z1.

Comparative Example 2

[0053] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
aluminum phosphate was uniformly dispersed and attached to the surface of
lithium cobaltate by using aluminum nitrate nonahydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of aluminum phosphate on an
elemental aluminum basis relative to lithium cobaltate was 0.012 mass %.
On a molar basis, the amount of aluminum phosphate was the same as that
of neodymium phosphate in Example 1.

[0054] The thus-produced battery is hereinafter referred to as a
comparative battery Z2.

Comparative Example 3

[0055] A battery was produced by the same method as that in Example 1,
except that a positive electrode active material was used in which
lanthanum phosphate was uniformly dispersed and attached to the surface
of lithium cobaltate by using lanthanum nitrate hexahydrate instead of
neodymium nitrate hexahydrate when a phosphate compound was deposited on
the surface of lithium cobaltate. The ratio of lanthanum phosphate on an
elemental lanthanum basis relative to lithium cobaltate was 0.057 mass %.
On a molar basis, the amount of lanthanum phosphate was the same as that
of neodymium phosphate in Example 1.

[0056] The thus-produced battery is hereinafter referred to as a
comparative battery Z3.

Experiment

[0057] Charge and discharge and continuous charge at high temperature were
performed on the invention batteries A1 to A6 and the comparative
batteries Z1 to Z3 under the conditions below. The residual capacity
ratio represented by formula (1) below was calculated and the results are
shown in Table 1.

[0058] In each of the batteries, a constant-current charge was performed
at a current of 1.0 It (750 mA) until the battery voltage reached 4.4 V,
and then a charge was performed at a constant voltage of 4.4 V until the
current reached 1/20 It (37.5 mA). After each of the batteries was left
to stand for 10 minutes, a constant-current discharge was performed at a
current of 1.0 It (750 mA) until the battery voltage reached 2.75 V to
measure the discharge capacity (discharge capacity before a continuous
charge test). After each of the batteries was left in a
constant-temperature oven at 60° C. for 1 hour, a constant-current
charge was performed at a current of 1.0 It (750 mA) at the same
temperature of 60° C. until the battery voltage reached 4.4 V, and
then a charge was performed at a constant voltage of 4.4 V for 60 hours.
After each of the batteries was taken out of the 60° C.
environment and cooled to room temperature, a constant-current discharge
was performed at a current of 1.0 It (750 mA) until the battery voltage
reached 2.75 V to measure the discharge capacity (first discharge
capacity after a continuous charge test). The residual capacity ratio was
then calculated using formula (1) below.

[0059] The residual capacity ratio indicates the degree of degradation of
a battery that is exposed to high temperature in a state of charge. A
battery has better thermal stability as the value increases.

[0060] As is clear from Table 1, in the invention batteries A1 to A6 that
each use the positive electrode active material in which the surface of
lithium cobaltate was coated with a surface treatment layer composed of a
phosphate compound of Nd, Sm, Eu, Er, Yb, or Lu, the residual capacity
ratio after the continuous charge was 87.8 to 88.9%, which are
significantly higher than that of the comparative battery Z1 (the
residual capacity ratio after the continuous charge was 80.3%) that used
a positive electrode active material in which the surface of lithium
cobaltate was not coated with a surface treatment layer. That is, in the
invention batteries A1 to A6, the positive electrode active material was
less degraded.

[0061] As is clear from publicly known documents, the comparative
batteries Z2 and Z3 use the positive electrode active material in which
the surface of lithium cobaltate were coated with a surface treatment
layer composed of aluminum phosphate or lanthanum phosphate,
respectively. The residual capacity ratio of the comparative batteries Z2
and Z3 after the continuous charge was 81.6% and 81.2%, respectively,
which are slightly higher than that of the comparative battery Z1 but
significantly lower than those of the invention batteries A1 to A6.

[0062] The reason for these results is unclear. However, in the case of
the invention batteries A1 to A6, part of the surface of lithium
cobaltate is coated with a surface treatment layer composed of the
phosphate compound selected in the present invention. Therefore, the
reaction area between lithium cobaltate and the electrolyte is decreased.
In addition, it is believed that such a surface treatment layer composed
of the phosphate compound of Nd or the like specifically exhibits an
anticatalyst effect, thereby suppressing the reaction between the
positive electrode active material and the electrolyte.

[0063] In contrast, in the case of the comparative batteries Z2 and Z3,
the reaction area between lithium cobaltate and the electrolyte is
decreased. However, the surface treatment layer composed of aluminum
phosphate or lanthanum phosphate does not exhibit an anticatalyst effect
and thus the reaction between the positive electrode active material and
the electrolyte is not suppressed.

[0064] Accordingly, it is believed that, in the case of the invention
batteries A1 to A6, since the reaction area between the positive
electrode active material and the electrolyte can be decreased while at
the same time the reaction therebetween can be suppressed, the
above-described experiment results were obtained.

[0065] Herein, neodymium, samarium, europium, erbium, ytterbium, and
lutetium used for the phosphate compounds exemplified in the First
Example are all rare-earth elements and have atomic numbers of 60 (Nd) to
71 (Lu). Since gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium
(Ho), and thulium (Tm) are also all rare-earth elements and have atomic
numbers of 60 (Nd) to 71 (Lu), Applicants believe that phosphate
compounds of these also produce the same effects.

Second Example

Example

[0066] A battery was produced by the same method as that in Example 4 of
the First Example, except that the ratio of erbium phosphate on an
elemental erbium basis relative to lithium cobaltate was increased to
0.17 mass % when the surface of lithium cobaltate was coated with a
surface treatment layer composed of erbium phosphate.

[0067] The thus-produced battery is hereinafter referred to as an
invention battery B.

Experiment

[0068] Charge and discharge and storage were performed on the invention
battery B under the conditions described in the First Example. The
residual capacity ratio represented by formula (1) above was calculated
and the results are shown in Table 2. Table 2 also shows the results of
the invention battery A4 and the comparative battery Z1.

[0069] In the invention battery B in which the ratio of erbium phosphate
on an elemental erbium basis relative to lithium cobaltate was 0.17 mass
%, the residual capacity ratio after the continuous charge was 88.8%,
which is substantially equal to the residual capacity ratio of the
invention battery A4 in which the ratio was 0.070 mass % on an elemental
erbium basis. Obviously, this value is significantly higher than that of
the comparative battery Z1 that used the positive electrode active
material in which a surface treatment layer was not formed on lithium
cobaltate. Therefore, it is understood that when the ratio of erbium
phosphate on an elemental erbium basis relative to lithium cobaltate is
in the range of 0.070 mass % or more and 0.17 mass % or less, the
residual capacity ratio is high and the degradation of the positive
electrode active material is suppressed.

[0070] Furthermore, it was found through the detailed examination
performed by the inventors of the present invention that the ratio of
erbium phosphate on an elemental erbium basis relative to lithium
cobaltate was preferably in the range of 0.010 mass % or more and 0.25
mass % or less. When the ratio is less than 0.010 mass %, the content of
erbium phosphate is excessively low and thus the effects achieved by its
addition cannot be sufficiently produced. On the other hand, when the
ratio is more than 0.25 mass %, the effects achieved by its addition can
be sufficiently produced. However, the effects are almost the same as
those in the case where the ratio is 0.25 mass %. Moreover, the interface
resistance is increased and thus the load characteristics may be
degraded.

[0071] This tendency is not limited to only erbium phosphate, and
Applicants believe that there is the same tendency as long as phosphate
compounds of neodymium, samarium, europium, gadolinium, terbium,
dysprosium, holmium, thulium, ytterbium, and lutetium are used. The
reason for this is as follows. As described in the First Example, the
residual capacity ratio is improved by employing a phosphate compound
other than erbium phosphate. Therefore, similarly to the case of erbium
phosphate, it is believed that when the content of the phosphate compound
is excessively low, the effects achieved by the addition cannot be
sufficiently produced, and, when the content of the phosphate compound is
excessively high, the interface resistance is increased and thus the load
characteristics may be degraded.

[0072] Moreover, the same applies even if a positive electrode active
material other than lithium cobaltate is used.

[0073] The present invention can be applied to, for example, driving power
supplies of mobile information terminals such as cellular phones,
notebook computers, and personal digital assistants (PDAs) and driving
power supplies of high-power machines such as electric vehicles and power
tools.

[0074] While detailed embodiments have been used to illustrate the present
invention, to those skilled in the art, however, it will be apparent from
the foregoing disclosure that various changes and modifications can be
made therein without departing from the spirit and scope of the
invention. Furthermore, the foregoing description of the embodiments
according to the present invention is provided for illustration only, and
is not intended to limit the invention.